1. Field of the Invention
The present invention is in the fields of medicine, immunology, virology and molecular biology.
2. Related Art
Vaccination has provided one of the most effective ways of fighting infectious diseases and has led to the most significant benefits for public health in the last century. Early vaccination strategies used live, attenuated or inactivated pathogens as the immunogen. Safety concerns within the public and the authorities have fostered a search for more defined and safer vaccines.
This search stimulated a new direction of research, where individual antigens were isolated or recombinantly expressed and injected as immunogens. Examples of these include the development and use of subunit vaccines. Such vaccines, however, often require the addition of an adjuvant to generate a sufficient immune response against the antigen, as an isolated protein is typically not sufficiently immunogenic to generate a protective immune response. Although several strong adjuvants are known, such as complete Freund's adjuvant, they are generally toxic and cannot be used in humans. Great efforts are therefore being made in the search for new adjuvants.
Recently, research on the principles of discrimination by the immune system between self and foreign has revealed that the degree of organization and the repetitiveness of the antigens on the surfaces of viruses are a very strong signal for an antigen to be recognized as foreign (Bachmann & Zinkernagel, Immunol. Today 17:553–558 (1996)). This property of viral structures was made use of in the design of new vaccines based on virus-like particles (VLPs), which combined the immunogenicity of viral structures and the improved safety profile of non-replicable vaccines. In those vaccines, the antigen is either fused or chemically attached to virus-like particles, the chemical attachment being covalent or non-covalent. Thus, the immunogenic property of the viral structure is transferred to the antigen by linking the antigen to virus-like particles.
A variety of VLPs have been used for the attachment of antigens. For example, WO 00/32227 describes the use of Hepatitis B core antigen in the production of certain types of vaccines.
A new class of highly expressable and highly immunogenic VLPs has been disclosed in WO 02/056905, which is incorporated herein by reference in its entirety. These VLPs are composed of the coat protein of RNA bacteriophages. The coat proteins are expressed recombinantly in bacteria, and the VLP does not contain the phage RNA genome and therefore cannot replicate.
A new RNA bacteriophage, AP205, has been recently identified (Klovins, J., et al., J. Gen. Virol. 83: 1523–33 (2002).) The AP205 RNA phage (Taxonomy ID: 154784) is a single-stranded, positive-strand RNA (no DNA stage) virus, which belongs to the Leviviridae family, Levivirus genus, Unclassified Levivirus subgroup. The other members of this subgroup are RNA phages BO1, fr1, TW19, and PP7. Two described Levivirus subgroups include following RNA phages: fr, JP501, f2, M12, MS2, and R17 (subgroup I) and BZ13, JP34, TH1, GA, and KU1 (subgroup II). The AP205 genome is 4267 nucleotides (nt) in length. Full-length genomic sequence: accessions AF334111, NC—002700. The natural host of the AP205 phage is Acinetobacter spp. (Klovins, J., et al., J. Gen. Virol. 83: 1523–33 (2002)). The genome of the AP205 phage comprises three large open reading frames (ORFs), which code for the maturation, the coat and the replicase proteins. In addition, two additional small ORFs are present at the 5′ terminus, preceding the maturation gene. The function of the proteins coded by these ORFs is unknown. It has been postulated that one of these ORFs might code for a lysis protein (Klovins, J., et al., J. Gen. Virol. 83: 1523–33 (2002)).